It is a requirement in certain processes to have the ability to measure the thickness of the insulating coating on a metallic product in the form of a sheet, rod, wire or the like.
A typical example of such processes is the production of sheet metal work requiring the application of a coating of plastics material to prevent rusting or deterioration of the metallic sheet when used in exposed conditions.
Another well known example is in the production of electrical cables by extrusion techniques. Electrical cables consist of a conductor core made of metallic electrically conducting material coated with a plastics insulating jacket for protective purposes including bodily contact with the live wire.
A convenient method exists for performing the measurement of an insulating coating on an electrically conducting material consisting of a combination of inductive and optical means.
In this system an inductive core is provided with a winding carrying an electrical current. This produces a magnetic field which threads the work product.
The current flowing through the coil of the inductor changes in response to the proximity of the conducting material of the product and this variation can be used to provide constant monitoring of the thickness of the coating.
This prior system is subject to errors created by the distance between the position of interaction of the magnetic flux of the inductor with the conducting material of the product and the position of impingement of the laser beam on the product, and also due to the linear translation of the product in the case of cable extrusion.